1. Field of the Invention
The present invention relates to a piezoelectric transformer used in power supply circuits for illumination such as cold-cathode fluorescent tubes or in various types of power supply circuits for generating high voltage, and particularly relates to the construction of a piezoelectric transformer requiring compact size, high efficiency, a high step-up voltage, and high reliability.
2. Description of the Related Art
In the prior art, coiled electromagnetic transformers have been used as transformer elements that generate high voltage in the illumination power supplies of cold-cathode fluorescent light tubes used in liquid crystal backlights or in power supply circuits in devices that require high voltage such as television deflection devices and charging devices for copiers. Such electromagnetic transformers are of a construction in which a conductor is coiled around a magnetic core, a large number of coiled conductors being needed to realize a high transformation ratio. A compact and highly efficient electromagnetic transformer has therefore been extremely difficult to achieve.
Piezoelectric transformers that take advantage of the piezoelectric effect have been proposed to replace such electromagnetic transformers (for example, C. A. Rosen: "Ceramic Transformer" in Proceedings of the Electronic Component Symposium (1957) pp. 256-211). FIG. 1 shows a perspective view of one example of a Rosen-type piezoelectric transformer, which is a typical piezoelectric transformer. Referring to FIG. 1, this transformer is constructed such that the length of piezoelectric ceramic plate 810 having the form of a long plate is equally divided between driver 81 and generator 82. Driver 81 is polarized in the direction of thickness of piezoelectric ceramic plate 810, and electrodes 811 and 812 (not shown in the figure) are formed over the entire region of driver 81 on both the upper and lower surfaces. Generator 82 is polarized in the longitudinal direction of piezoelectric ceramic plate 810 and electrode 815 is formed on the end surfaces perpendicular to the longitudinal axis.
To boost voltage with this piezoelectric transformer, an alternating current voltage is applied from the outside across the two electrodes 811 and 812 above and below driver 81, i.e., across input-output terminal 817 and input terminal 818. Driver 81 oscillates in the longitudinal direction due to the horizontal piezoelectric effect in accordance with the above-described alternating current input voltage. A vibration is therefore generated in piezoelectric ceramic plate 810 in the longitudinal direction, and due to the vertical piezoelectric effect caused by this vibration, a stepped-up voltage of the same frequency as the input voltage is generated at generator 82 between electrode 815 at the end surface of the generator and electrode 811 or electrode 812 of driver 81 (in FIG. 1, between electrode 812 and electrode 815), i.e., between input/output terminal 817 and output terminal 819. In this case, an extremely high output voltage can be obtained if the frequency of the above-described alternating current input voltage is made the same as the mechanical resonance frequency in the longitudinal direction of piezoelectric ceramic plate 810. The piezoelectric transformer shown in FIG. 1 drives the above-described resonance in a primary mode, and therefore the length L of piezoelectric ceramic plate 810 and the wavelength .lambda. of the alternating current input voltage are set such that L=.lambda./2.
In the above-described Rosen-type piezoelectric transformer of FIG. 1, the input/output voltage ratio (step-up rate) can be made larger if a high-impedance load approaching open is connected to generator 82, but a step-up rate of only several times to several tens of times can be obtained if a low-impedance load (for example, on the order of 100 k.OMEGA.) is connected. A cold-cathode fluorescent tube used as the backlight of a liquid crystal characteristically has high impedance when first turned on, but after lighting up, the impedance drops to a level of 100 k.OMEGA.. As a result, when used to light up a cold-cathode fluorescent tube, the above-described Rosen-type piezoelectric transformer obtains a sufficient step-up rate when first lighting up, but then enters a low-impedance state after initial light-up and therefore fails to obtain sufficient step-up rate, giving rise to flickering or other illumination state problems. To solve theses problems, another problem occurs which necessitates the addition of a coil transformer for preliminary step-up before the input of the piezoelectric transformer.
Piezoelectric transformers that have solved the problem of this step-up rate are disclosed in Japanese Patent Laid-open No. 83035/97 and Japanese Patent Laid-open No. 83036/97. FIG. 2a shows an exploded perspective view of the piezoelectric transformer described in Japanese Patent Laid-open No. 83035/97, and FIG. 2b is a front view showing the electrically connected and polarized states. FIG. 3a shows an exploded perspective view of the piezoelectric transformer described in Japanese Patent Laid-open No. 83036/97, and FIG. 3b is a front view showing the electrical connection and polarized states.
Explanation is first presented with reference to FIGS. 2a and 2b regarding the piezoelectric transformer disclosed in Japanese Patent Laid-open No. 83035/97. As shown in FIG. 2a, driver piezoelectric ceramic plate 902 having input electrodes 904a and 904b formed on the upper and lower surfaces, respectively, and driver piezoelectric ceramic plate 903 having input electrodes 905a and 905b formed on the upper and lower surfaces, respectively, (input electrodes 904b and 905b not shown) are bonded on opposite sides of the central portion of generator piezoelectric ceramic plate 900 having output electrodes 901a and 901b (output electrode 901b not shown) formed on the two end faces. In a piezoelectric transformer that is bonded and formed in this way, driver piezoelectric ceramic plates 902 and 903 and the central portion of generator piezoelectric ceramic plate 900 are polarized in the direction of thickness as indicated by the arrow of polarization direction 906a in FIG. 2b, and both ends of generator piezoelectric ceramic plate 900 are polarized in the direction of length as indicated by the arrows of polarization direction 906b.
After connecting input electrodes 904a and 905a to the construction polarized in this way, input terminal 907a is then connected to input electrodes 904a and 905b as shown in FIG. 2b. Then, after connecting input electrodes 904b and 905b, input terminal 907b is connected. Output electrode 901a is then connected to output terminal 908a, and output electrode 901b is connected to output terminal 908b. Application of an alternating current voltage across input terminals 907 and 907b then causes driver piezoelectric ceramic plates 902 and 903 to oscillate, and generator piezoelectric ceramic plate 900 accordingly oscillates in the direction of length due to the piezoelectric horizontal effect, thereby generating a stepped-up voltage of the same frequency as the input voltage between output terminals 908a and 908b.
When rating the step-up rate by connecting a low-impedance load of about 100 k.OMEGA. to this piezoelectric transformer of the prior art, a step-up rate of from 18-25 times is obtained when the thickness of generator piezoelectric ceramic plate 900 is 0.5-0.2 mm.
Explanation is next presented with reference to FIGS. 3a and 3b regarding the piezoelectric transformer disclosed in Japanese Patent Laid-open No. 83036/97. This piezoelectric transformer is substantially the same as the above-described piezoelectric transformer disclosed in Japanese Patent Laid-open No. 83035/97, the only differences being the connected states and polarization directions of the drivers and generator. As shown in FIG. 3a, driver piezoelectric ceramic plate 1012 having input electrodes 1014a and 1014b formed on the upper and lower surfaces and driver piezoelectric ceramic plate 1013 having input electrodes 1015a and 1015b formed on the upper and lower surfaces (input electrodes 1014b and 1015b not shown) are bonded on opposite sides of the central portion of generator piezoelectric ceramic plate 1010 having output electrodes 1011a and 1011b (output electrode 1011b not shown) formed on the two end faces. In a piezoelectric transformer that is bonded and formed in this way, driver piezoelectric ceramic plates 1012 and 1013 and the central portion of generator piezoelectric ceramic plate 1010 are polarized in the direction of thickness as indicated by the arrows of polarization direction 1016a in FIG. 3b, and both ends of generator piezoelectric ceramic plate 1010 are polarized in the direction of length as indicated by the arrows of polarization direction 1016b.
After connecting input electrodes 1014a and 1015b together, input terminal 1017a is then connected to the construction polarized in this way as shown in FIG. 3b, and input terminal 1017b is connected after input electrodes 1014b and 1015a are connected. Output electrode 1011a is then connected to output terminal 1018a, and output electrode 1011b is connected to output terminal 1018b. Application of an alternating current voltage across input terminals 1017a and 1017b then causes driver piezoelectric ceramic plates 1012 and 1013 to oscillate, and generator piezoelectric ceramic plate 1010 accordingly oscillates in the direction of length due to the piezoelectric horizontal effect, thereby generating a stepped-up voltage of the same frequency as the input voltage between output terminals 1018a and 1018b.
When rating the step-up rate by connecting a low-impedance load of about 100 k.OMEGA. to this piezoelectric transformer of the prior art, a step-up rate of from 11-20 times is obtained when the thickness of the generator piezoelectric ceramic plate is 0.5-0.2 mm.
Although the step-up rate of the piezoelectric transformer disclosed in Japanese Patent Laid-open No. 83036/97 is somewhat lower than that of the piezoelectric transformer disclosed in Japanese Patent Laid-open No. 83035/97, both obtain a higher step-up rate than the Rosen-type piezoelectric transformer of FIG. 1, and in particular, both achieve an improvement in step-up rate at a low-impedance.
The Rosen-type piezoelectric transformer and the prior-art piezoelectric transformers disclosed in Japanese Patent Laid-open No. 83035/97 and Japanese Patent Laid-open No. 83036/97 all employ vertical piezoelectric oscillation in the direction of length. Accordingly, the positions of antinodes and nodes resulting from this vertical oscillation and the positions at which the input/output electrodes are drawn out are closely connected with hindrances of oscillation, and affect the energy-conversion efficiency as well as the reliability of electrode-to-lead connections of the piezoelectric transformer.
In the piezoelectric transformers of these constructions, output electrodes 901a, 901b (1011a and 1011b) are on the end faces of generator piezoelectric ceramic plate 900 (1010), as with the Rosen-type transformer of FIG. 1. In this case, both end faces in the direction of length coincide with the antinode of mechanical resonance, and output electrodes 901a and 901b (1011a and 1011b) are formed at the positions of the antinode of this mechanical resonance. Accordingly, the points on output electrodes 901a and 901b (1011a and 1011b) that connect with output terminals 908a and 908b (1018a and 1018b) are inevitably located at antinodes of mechanical resonance in the longitudinal direction of generator piezoelectric ceramic plate 900 (1010). The antinode of vibration is located at a point having large displacement, and problems will inevitably result if a connection structure on the electrode such as solder between a lead wire and output electrodes 901a and 901b (1011a and 1011b) is at the antinode of vibration.
Specifically, the connection structure on the electrode impedes vibration of piezoelectric ceramic plate 900 (1010), leading to a decrease in transformer efficiency as well as a decrease in the step-up rate. There is the additional problem that lead wires connected at the antinodes of vibration undergo excessive vibration and break. Providing a connection point of an electrode at the antinode of mechanical resonance thus entails losses in transformer efficiency, step-up rate characteristics, and connection reliability.
The position of the output electrode at the antinode of mechanical resonance is next described with reference to FIG. 4.
FIG. 4a is a section of the prior-art piezoelectric transformer described in Japanese Patent Laid-open No. 83035/97, FIG. 4b shows the distribution of displacement of vibration, and FIG. 4c shows the distribution of stress of vibration (these figures illustrate cases of vibration in tertiary mode).
It can be seen from FIG. 4b and FIG. 4c that the end faces are at the antinodes of vibration (regardless of the vibration mode) and that nodes of vibration are located at the center and at points 1/6 the length of the ceramic plate from the end faces. Considering the relation between the positions shown herein and the positions at which electrodes are drawn out in the prior-art piezoelectric transformers (These figures take as an example the structure of the piezoelectric transformer described in Japanese Patent Laid-open No. 83035/97, but the other case is equivalent.), lead-outs from input electrodes 904a, 904b, 905a, and 905b to input terminals 907a and 907b can be positioned at the vibration nodes indicated by arrows 71. However, output terminals 908a and 908b that draw out from output electrodes 901a and 901b are positioned at the antinodes of vibration as indicated by arrows 72. Antinodes of vibration are positions of great displacement, and connecting lead wires by soldering at such positions not only impedes vibration, thereby decreasing the energy-conversion efficiency of the piezoelectric transformer, but also places a burden on the lead wires due to vibration, with the danger of line breakage.
In the constructions of the piezoelectric transformers described in Japanese Patent Laid-open No. 83035/97 and Japanese Patent Laid-open No. 83036/97, sintered monoplates are used as driver piezoelectric ceramic plates 902 and 903 (1012 and 1013), and the input capacitance is increased and the step-up rate heightened by making these sintered monoplates thin. However, these are naturally limited in varying the input capacitance and adjusting step-up rate by merely changing the thickness of a sintered monoplate. Higher step-up rates, for example, necessitate even thinner plates, but limits are imposed by the difficulties of manipulating extremely thin plates and by the effect of vibration on lead wire connections, and these factors ultimately impose an upper limit on the step-up rate.
In the constructions of the piezoelectric transformers described in Japanese Patent Laid-open No. 83035/97 and Japanese Patent Laid-open No. 83036/97, moreover, driver piezoelectric ceramic plates 902 and 903 (1012 and 1013) must be positioned and bonded in the center of generator piezoelectric ceramic plate 900 (1010). This positioning affects the vibration mode and must therefore be achieved with extreme accuracy, thereby requiring more processes for positioning during assembly and ultimately resulting in higher costs.